Add support for CMock unit testing framework

[BrunoMPires-zz]

Since pio test seems to be based on ThrowTheSwitch's Unity, it would be great also being able to use their CMock mocking framework, specially when compiling for platform native when a bunch of dependencies will fail to compile with native gcc.

[bobbyflowstate]

Is there a way to currently mock things in PlatformIO?

[dangarbri]

This would be really good to have moving forward. It's difficult to have a reliable process when CI can't properly test FW code.